A hardened body of cement always undergoes a sharp reduction in mechanical strength when heated at a high temperature of, say, 450.degree. C. or above. It is, for example, known that, if a hardened body of Portland cement is heated at 450.degree. C., the calcium hydroxide formed by hydration of calcium silicate is decomposed into calcium oxide and water, resulting in the occurrence of heavy shrinkage in the body of cement, which sharply reduces its mechanical strength, particularly compressive strength (Yu Harada & Toru Sakai: "Heat Resistant Concrete," Industry & Products, 50, page 119, right column). It is also known that a hardened body of aluminous cement has its mechanical strength reduced sharply when heated at a high temperature, though for different chemical reasons from those hereinabove described with respect to Portland cement [Akira Wakabayashi: "Properties of a Hardened Body of Aluminous Cement at a High Temperature," Journal of the Society of the Ceramic Industry, 69, 2 (1961), page 28]. Accordingly, nobody has hitherto thought of burning a hardened body of cement at a high temperature in order to obtain a molded product of cement having improved strength.
Under the circumstances, the smoking treatment, which is often utilized in the manufacture of a molded clay product in order to impart a beautiful tone of color and luster to its surface has hitherto been considered inapplicable to a molded product of cement, because the high temperature required for the smoking treatment reduces the strength of the cement product. The same reason has inevitably imposed various restrictions on the glazing of a molded product of cement. It has been imperative to use the glaze which completely melts at a temperature under 450.degree. C., in order to avoid any reduction in the mechanical strength of a hardened body of cement when it is heated for the fusion of the glaze. In order to cope with the situation, there has been developed a phosphoric frit glaze having a low melting point as disclosed in, for example, Japanese Patent Publication No. 10530/1974. This special glaze is, however, not only inferior in weather and chemical resistance to an ordinary glaze having a high melting point, but is also very expensive, and is incapable of imparting a very beautiful surface finish to a product of cement.
It is an object of this invention to provide a method of manufacturing a molded product of cement having a higher mechanical strength than any such product known in the art, by incorporating between a preliminary hydration hardening process and a full-scale hydration hardening process a process for burning a hardened body of cement at a high temperature.
It is another object of this invention to provide a method of manufacturing a product of cement having a surface on which a beautiful and lustrous film of carbon is formed by smoking treatment.
It is a further object of this invention to provide a method of manufacturing a product of cement having a beautifully glazed surface formed by a glaze having a high melting point.
Other objects of this invention will become apparent from the following description.